The global positioning system (GPS) is being developed and deployed by the U.S. Department of Defense to support military navigation and timing needs. Federal Aviation Administration now has declared that the GPS system is acceptable for civil aircraft navigation. The GPS provides a most competent and all-weather system in navigation and surveying. It consists of at least 18 satellites and 3 active spare satellites. There may be 24 GPS satellites in the final constellation in order to eliminate the degradation.
These satellites transmit at frequencies L1=1575.42 MHz and L2=1227.6 MHz with two types of modulated codes, which are precision (P) code and coarse/acquisition (C/A) code with the chipping rates of 10.23 MHz and 1.023 MHz respectively. The encrypted P-code provides the precise positioning service to the United states and allied militaries. The standard positioning service is available world wide with degraded signals in comparison with the precise positioning service. The degradation, which is called as selective availability, is through the introduction of clock errors to the on-board atomic clocks and to the navigation message transmitted by the satellites. Civil GPS receivers are C/A code receivers providing the positioning accuracy at 100 m level.
A requirement in the positioning accuracy better than 100 m leads to the needs of supporting sites to supplement the information provided by the standard positioning service. The differential GPS uses a known supporting site as a reference. Pseudo ranges to all tracked satellites from the known site are observed through the aid of a GPS receiver. The observed pseudo ranges are compared with the degraded pseudo range information from the GPS navigation message. Differences between the observed and degraded pseudo ranges reveal the errors associated with the selective availability and ionospheric effects. The motivation for using differential operation is that these errors are common to GPS receivers operating at the spatial and temporal vicinities of the known site. The revealed errors, which are referred to as differential corrections, then are transmitted over a digital data link to the users at the vicinity to remove their GPS receiver degradation owing to the selective availability and ionospheric effects.
Supporting sites may transmit GPS like signals. These signals are referred to as the pseudolites. Their use leads to carrier-phase tracking in sharping the GPS accuracy for precision aircraft landing at lower visibility Category 2 and Category 3 conditions. The accuracy will further enhanced the reliability in using the GPS system. The dependability and accuracy will be properly assured, if more known and widely separated sites are deployed.
GPS signal generators and receivers are active radio frequency (RF) systems. Each of these supporting sites is equipped with the active systems as well as stable clocks. The operating foundations of the GPS system are the spread spectrum and precision time reference. A stable clock is an essential apparatus for a GPS signal generator and receiver to execute their functions. The stable clocks have to be strictly maintained and rigidly synchronized. The integrity of these active systems has to be stringently preserved. The synchronization and integrity are most important to achieve the desired accuracy in a GPS application. Any failure of an active RF signal generator or receiver at a supporting site has to be promptly identified and isolated in order to prevent any catastrophe arising from the failure. It is difficult as well as expensive to maintain the supporting sites at such high integrity and expectation. The difficulty mounts, if a secondary supporting site is not directly visible to the master supporting site. No low cost means exist to promptly verify the integrity of the transmitted GPS signals under such circumstances.
In light of the above, there is a need in the art for a supporting network which is simple and inexpensive and does not require active RF signal generators and receivers located at individual supporting sites. There are further needs to provide a low cost service to wide segments of users with precision navigation and time information despite the selective availability. Furthermore, there are needs to advance the art of providing precision navigation and time.